Aeration and mixing have been used for treating water and other liquids for over a century. During that time various methods, including the following, have been employed:
Compressor/diffusers use a suitable compressor to force gas below the liquid surface and through a diffuser. As the bubbles rise to the surface, gas is transferred from the bubbles to the liquid. Mixing is accomplished via the change in liquid density created by the air and the hydraulic resistance of the bubbles as they travel to the liquid surface. Diffuser types range from coarse bubble to fine bubble diffusers. Coarse bubble systems do not transfer oxygen as efficiently and can be energy-inefficient to operate, when compared to fine bubble systems. Fine bubble diffusers are at first more energy-efficient, but they can become fouled, clogged, or damaged, resulting in decreased air transfer. The fine-bubble diffusers, in particular, are limited in turn-down capability, due to increased fouling problems at lower gas flow rates.
U.S. Pat. No. 3,630,498 to Belinski shows the use of a small, high-speed rotating mixing and aerating element comprised of a pair of horizontal radially extending blades or foils. In operation, a partial vacuum is created in a zone of cavitation, which is formed behind the foils. Gas bubbles which emerge from the blades enter the zone of cavitation and expand due to the reduced pressure around the bubbles. While expanded, the bubbles are shattered by hydraulic forces into smaller bubbles. The shattered bubbles then exit the reduced pressure zone of cavitation and are further reduced in size as they are subjected to ambient pressure. Critical to the Belinski patent is the creation of the zone of cavitation. To create a zone of cavitation in a practical device, the foils must be short (such as 24 inches) and rotated at very high speeds (such as 450 RPM). Such a device is best suited for a smaller area. If the foils are made appreciably longer, the energy cost and physical loads of high-speed rotation quickly becomes prohibitive.
Surface Aerators use motors to drive impellers or blades near the surface. They either lift the water into the air, or aspirate air and inject it just below the surface. Surface aerators generally have a poor air transfer efficiency when compared to fine bubble diffused aeration systems. In other words they consume more horsepower hours of energy for each pound of dissolved oxygen they produce. In addition, mixing from surface aerators is generally limited to liquid near the surface. Also, mixing energy tends to be point loaded at or near the impeller. Localized zones of high shearing forces tend to damage delicate floc structures necessary for proper liquid clarification. Further, they are limited in the length of the shaft overhang, and have a limited shaft bearing life.
Turbine/Spargers aerators use compressors to force and distribute a gas under the liquid surface. They also use a submerged impeller located just above the diffuser (sparger) to shear the bubbles and provide bulk mixing. Disadvantages of turbine spargers are similar to those for surface aerators with the additional disadvantage that the turbine sparger needs a source of compressed gas such as a compressor.
Jet Aerators use a liquid pump and an eductor to entrain gas into the liquid using the Venturi principle, as in U.S. Pat. No. 4,101,286. Jet aerators may be equipped to mix additional gas, liquid, or solid chemicals into the bulk liquid. They are reliable, have good turn down capability, and tend to be good mixers; however, they are inefficient aerators.
Blade Diffusers as taught in Ingram U.S. Pat. No. 1,383,881 (issued Jul. 5, 1921) use a flotation apparatus having rotating blades that dispense gas bubbles into a body of liquid. The design of these blades is dictated, however, by the requirement that they also act as impellers to rotate the blades as well as discharging the gas bubbles. The blades are pitched so that the leading edges are elevated about 45 degrees. As a result, the emerging gas is formed into elongated and then enlarged bubbles, which provide less efficient introduction of the gas into the liquid. In addition, examination of the patent and some research indicates that the blades would rotate in the opposite direction than is indicated in the Ingram Patent. This would result from the upward flow of fluid caused by the fluid lift pump effect of the released gas moving upward toward the liquid surface. Such vertical water flow across the pitched blades would appear to in fact cause rotation opposite that which is indicated in the patent.
Another excellent example of a device for aeration and mixing of large bodies of liquid is taught in U.S. Pat. No. 5,681,509, which teaches an apparatus and method for mixing and introducing gas into a large body of liquid by rotating a plurality of permanently mounted spoke-like discharge members which are below the surface of the liquid body. These members have upwardly facing perforated discharge surfaces through which compressed gas is released up into the liquid. Upward lift is countered by angling the members which are tilted with their leading edges lower than their trailing edges and balancing the rotation speed to achieve substantially zero lift. A control system is provided to change the depth of submergence of the discharge members to regulate dissolved gas infusion rate and speed of member rotation to maintain angle of attack. U.S. Pat. No. 5,681,509 teaches the use of permanently mounted blade members which are self supporting for the load forces encountered and which can prove labor intensive to change if needed, and also teaches the use of a vertically inclining main shaft which, while providing valuable utility in the ability to raise the blade members from the liquid in which they rotate, does require a substantial frame and mechanical structure to support the components allowing for the inclining main shaft.
Of course, the discharge members which have surfaces through which compressed gas may be discharged can face the risk of damage should the air pressure in those members be interrupted. In that case, the higher liquid pressure outside the members could force the liquid into the discharge members, potentially carrying undesirable particulates with it and thereby damaging/clogging the discharge members. U.S. Pat. No. 6,808,165 B1 discloses one advantageous structure for preventing such damage, in which the discharge members (diffuser blades) are attached to a hub mounted on a main shaft that automatically cantilevers out of the fluid should compressed gas supplied to the diffuser blades through the main shaft cease.
The present invention is directed toward overcoming one or more of the problems set forth above.